PHP - 浮点数精度 [英] PHP - Floating Number Precision

问题描述

  $ a ='35'; 
 $ b ='-34.99'; 
 echo（$ a + $ b）; 
  

结果为0.009999999999998

接着就，随即？为什么PHP不会返回预期的0.01？

>解决方案

因为浮点运算！=实数运算。由于不精确性的差异，对于一些花车 a 和 b ，（ a + b）-b！= a 。这适用于使用浮动的任何语言。

由于浮点数是二进制有限精度的数字，有限数量的可表示的数字，精确度问题和这样的惊喜。这是另一个有趣的阅读：计算机科学家应该知道什么关于浮点运算

一>。

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回到你的问题，基本上没有办法准确地表示二进制的34.99或0.01（就像十进制， 1/3 = 0.3333 ...），所以使用近似值。为了解决这个问题，你可以：

1. 使用 round（$ result，2） 将结果舍入到小数点后两位。
   

2. 使用整数。如果是货币，例如美元，那么存储35.00美元为3500和34.99美元，然后将结果除以100。
   



可惜的是，PHP没有十进制数据类型，如其他 languages 。



$a = '35';
$b = '-34.99';
echo ($a + $b);

Results in 0.009999999999998

What is up with that? I wondered why my program kept reporting odd results.

Why doesn't PHP return the expected 0.01?

解决方案

Because floating point arithmetic != real number arithmetic. An illustration of the difference due to imprecision is, for some floats a and b, (a+b)-b != a. This applies to any language using floats.

Since floating point are binary numbers with finite precision, there's a finite amount of representable numbers, which leads accuracy problems and surprises like this. Here's another interesting read: What Every Computer Scientist Should Know About Floating-Point Arithmetic.

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Back to your problem, basically there is no way to accurately represent 34.99 or 0.01 in binary (just like in decimal, 1/3 = 0.3333...), so approximations are used instead. To get around the problem, you can:

1. Use round($result, 2) on the result to round it to 2 decimal places.

2. Use integers. If that's currency, say US dollars, then store $35.00 as 3500 and $34.99 as 3499, then divide the result by 100.

It's a pity that PHP doesn't have a decimal datatype like other languages do.

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